Silsilah as-Sahihah 3013 — How to Gain Angels’ Prayers; Dua in Absence

Abu Huraira (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When a person prays for (his Muslim brother) behind his back, the angel says (while praying for him): 'For you also, the same (be granted).'"